The invention relates to an arrangement that facilitates periodic tensioning of an endless cutting chain on a guide bar of a chain saw.
As the links in the cutting chain of a chain saw wear with use, the chain stretches and becomes loose on the guide bar. Many methods exist to move the guide bar longitudinally away from the chain saw body and drive sprocket to take the slack out of the links of the cutting chain to ensure that the links of the chain remain seated in and ride in a peripheral channel in the guide bar. A number of methods require the operator to physically move the guide bar longitudinally from the chassis to a tensioned position and then employ a tool or tools to physically tighten the assembly so that the bar will remain in the new position. In other methods, tightening screws, hydraulic pistons and other devices for moving the bar to its new position are employed and again the assembly is tightened, in many cases employing a tool or tools to ensure that the bar remains in the new position.
The purpose of the invention is to provide an arrangement that will enable the chain guide bar to move outwardly to the tensioned position automatically when the locking friction on the bar is relieved and to remain in the new position once the bar is tightened without the use of a tool or tools. Additionally, the invention can provide an indication to the chain saw operator when the chain should be replaced.
The invention provides a means of automatic tension adjustment of an endless cutting chain on a guide bar of a chain saw as the chain links expand with wear, without requiring the use of any tools to make the adjustment.
In accordance with the invention, the chain saw comprises, besides an engine body or chassis, a guide bar with an endless cutting chain, and a clutch cover assembly. The engine drives a chain drive sprocket. The guide bar is aligned in a horizontal position relative to the engine chassis, forward of the chain drive sprocket, by two studs affixed to and projecting perpendicularly from the side of the engine chassis. The guide bar is located in a manner to allow the drive sprocket to engage the links of the cutting chain. The studs are specifically located adjacent to each other in a horizontal plane and extend through a horizontal slot in the guide bar. The guide bar is of an elongated plate configuration that provides a channel around its periphery in which the links of the endless chain ride. A tensioner pin affixed to the guide bar and extending perpendicularly from it provides a surface that is engaged by a spring biased cam. The cam operates through the tensioner pin to continuously apply a force on the bar away from the drive sprocket. A locking plate with a slot coinciding with the slot in the guide bar is precisely located on the guide bar by tabs that project perpendicularly from the locking plate and extend through the slot in the guide bar. A hole in the locking plate aligns with the position of the tensioner pin on the guide bar and allows the tensioner pin to extend through the locking plate. An elongated high friction surface is coined or otherwise formed on the locking plate above the slot. When assembled, the high friction surface on the locking plate mirrors the location of a similar high friction surface coined or otherwise formed on a cover plate. The cover plate is attached to the clutch cover assembly by a machine screw and located in position to mirror the locking plate by locator pins molded on the inner face of the clutch cover assembly.
The clutch cover assembly is a housing molded or otherwise formed of a suitable material such as plastic or die cast metal that is attached to the chain saw motor body by a knob. The knob, having an internally threaded nut insert, is threaded onto the forward alignment stud affixed to the motor chassis and extending perpendicularly from it. The clutch cover assembly provides an internal molded cavity to house the tension spring that continuously biases the cam against the tensioner pin on the guide bar. The cam profile has a unique function of ensuring that the principle force vector applied to the tensioner pin is generally horizontal for improved automatic adjustment operation. Additionally, the cam configuration ensures that the cam is in a proper location before the clutch cover assembly can be fully seated and before the knob can be threaded onto the alignment stud. The cam is attached to a pivot pin on an override lever that extends through the clutch cover assembly. The override lever is fixed to the pivot pin externally of the clutch cover and rides in a channel molded on the outer face of the clutch cover assembly. As the override lever is directly attached to the cam, it follows the movement of the guide bar as it takes the slack out of, i.e. tensions, the cutting chain. Nomenclature embossed or otherwise labeled on the side of the override lever channel indicate to the operator when the cutting chain has reached its full extension and should be replaced.
The knob on the clutch cover assembly, when turned clockwise, allows the operator to tighten the cover plate high friction surface onto the locking plate high friction surface to lock the guide bar in proper operating position. Alternatively, when turned counter-clockwise, the knob releases the two friction surfaces to allow the spring operated cam to again move the guide bar forward towards its full extension and take any slack out of the links of the endless cutting chain. A knob lever that can be easily raised to a perpendicular position relative to the knob face provides an easily gripped element for forcibly turning to the knob.